In an attempt to make money from more parts of its service, Twitter is rolling out a new form of advertising called "promoted accounts."
In short, it will let advertisers buy a spot on Twitter's sidebar with a suggestion of Twitter accounts you should follow--that is, if it's relevant to accounts the user is already following.
Promoted accounts join trends, and individual tweets as items that can be paid for to gain more prominence on various parts of the site. Short of things like banner and box ads, which do not currently appear on Twitter's pages, these are now the main form of advertising.
Twitter says the promoted accounts program is live now, but users may not see it immediately since it's only being tested with "a handful of companies."